In the past, there have been proposed active type spatial information detection devices configured to detect spatial information by means of emitting light to a space to be searched and receiving light from the space. These kinds of spatial information detection devices detect the spatial information such as a distance to an object present in a space, a reflectance or an absorbance of an object present in a space, a reflectance or an absorbance of a medium in a space, and presence or absence of an object in a space. These spatial information detection devices are designed depending on types of the spatial information to be detected.
Known as the spatial information detection device detecting a distance to an object present in a space as the spatial information is a device using the time of flight measurement to measure time starting from time of emitting light and ending on time of receiving the light reflected by the object. The measured time is converted to the distance to the object.
For example, this kind of the spatial information detection device emits intensity-modulated light (hereinafter referred to as “signal light”) having an intensity oscillating at a constant period such as a sine wave, and measures a phase difference between a waveform of the intensity-modulated light at emitted time and a waveform of the intensity-modulated light at received time. Since the period of the waveform of the intensity-modulated light is constant, a distance to an object can be calculated on the basis of the measured phase difference (see e.g., document 1 [JP 2004-45304 A]).
For example, amounts of electric charges corresponding to amounts of received light are acquired at a plurality of timings synchronized with the signal light, and the phase difference is calculated by use of a relation between the amounts of the electric charges at the plurality of timings. Since this phase difference is corresponding to a time difference between time of emitting the signal light and time of receiving light reflected by the object, the time difference r between the time of emitting light and time of receiving the light is expressed by an equation of r=T(Ψ/2π), wherein T [s] denotes a period of the signal light and c [m/s] denotes the light speed and Ψ [radian] denotes a phase difference between the waveforms of the intensity-modulated light. Further, a distance L to the object can be calculated by use of an equation of L=(1/2)c*r=(1/2)c*T(Ψ/2π). For example, when the signal light has a frequency of 20 MHz, the period T is 50 [ns]. Hence, a measurable maximum distance (hereinafter referred to as “maximum measuring distance”) is 7.5 [m]. In summary, since the signal light having an intensity oscillating at a constant period is used, an upper limit of a measurable range is equal to a distance corresponding to a half period of the signal light (distance corresponding to a half wavelength).
Further, a device designed to provide a light projection period in which the signal light is emitted to the space and a non-light projection period in which no signal light is emitted to the space is known as the spatial information detection device detecting the spatial information such as a reflectance or an absorbance of an object present in a space, a reflectance or an absorbance of a medium in a space, and presence or absence of an object in a space. According to this device, an effect caused by environmental light and surrounding light (hereinafter referred to collectively as “environmental light”) is eliminated by use of a difference between the amounts of the received light respectively related to the light projection period and the non-light projection period, and detects only a component of reflection light derived from the signal light emitted to the space (see e.g., document 2 [JP 2006-121617 A]).
The amount of the received light in the non-light projection period is equal to the amount of the received light corresponding to only the environmental light but the amount of the received light in the light projection period is equal to the amount of the received light corresponding to the emitted signal light and the environmental light. Hence, the spatial information detection device eliminates a component corresponding to the environmental light with reference to the amount of the received light in the light projection period and the amount of the received light in the non-light projection period, and extracts a component corresponding to the reflection light derived from the signal light. In other words, the spatial information corresponding to the intensity of the reflection light derived from the signal light is detected.
In aforementioned documents 1 and 2, to receive light from the space, an imaging element is used. To calculate the distance, a distance image having pixel values defined by distance values is generated. To calculate the intensity of the reflection light, a grayscale image having pixel values defined by intensity values is generated.
As described above, according to the techniques disclosed in aforementioned documents 1 and 2, the spatial information is detected based on the reflection light derived from the signal light emitted to the space. Therefore, when a transparent object (e.g., glass) allowing light to pass is present between the spatial information detection device and the space to be searched for the spatial information, reflection light from the transparent object may be received in addition to light from the space to be searched for the spatial information. In this case, the reflection light from the transparent object is likely to be added to the light from the space. Thus, the amount of the received light may contain some components different from the component relating to the space. Therefore, there arises a problem that accurate detection of the spatial information may be prevented.
Further, even when not the transparent object but an object exists in a vicinity of the spatial information detection device, reflection light (mainly, diffuse reflection light) caused by the object may come into the spatial information detection device, and such reflection light may be added to the light from the space. In this case, the accurate detection of the spatial information may be prevented. In the above case, the vicinity of the spatial information detection device means a region between the spatial information detection device and the space to be searched for the spatial information.
As described above, when an object (hereinafter referred to as “unintended object”) which is not selected as a target for detecting the spatial information is present between the spatial information detection device and the space, reflection light from the unintended object is added to light from the space and comes into the spatial information detection device. Consequently, there arises a problem that the spatial information detection device may fail to detect the spatial information accurately.